Photo semiconductor devices are known to be useful in many applications including display devices, optical communications, optical disks, and image processing means, and many materials have been proposed for them.
For example, a photo semiconductor device was first developed for use in the infrared region using GaAs compounds. Later, AlGaAs, AlGaInP and other materials using a shorter wavelength were proposed. At a shorter wavelength, the emission is visible so that used for display. There exist red to green LED for display now. If blue LED is turned to practical use, full color display system using LED is realized. At the same time, the recording density of an optical disk and other information processing devices may be enhanced. Accordingly, as a photo semiconductor material using a group II-VI wide gap semiconductor (mixed crystal) compounds, for example as disclosed in Applied Physics 6.0 (1991), p.536, ZnS, ZnSe, ZnTe, CdS, CdSe, and mixed crystals combining these compounds have been mainly proposed.
Above all concerning the laser diode, in group III-V semiconductor, compounds, the laser diode is obtained in at least a part of the yellow region by using GaInP/AlInP, as disclosed in Electron. Lett. 26 (1990), p. 657. In group II-VI semiconductor compounds, the laser diode is obtained in at least part of the blue-green region, when converted to room temperature, by using a ZnCdSSe mixed crystal of a group II-VI semiconductor compounds, as disclosed in Appl. Phys. Lett. 60 (1992), p. 2045.
However, for the photo semiconductor, superior crystallinity is desired. From the viewpoint of using a lattice matching system, the problem is that the practical photo semiconductor element possessing a luminous wavelength from the blue to ultraviolet region cannot be obtained from existing material systems.
Besides, in a laser diode, no material is known to have a broad band offset in both the conduction band and the valence band, for the active layer for emitting a blue light, and a blue laser diode cannot be obtained.